Semiconductor device

ABSTRACT

An object is to achieve low-power consumption by reducing the off-state current of a transistor in a photosensor. A semiconductor device including a photosensor having a photodiode, a first transistor, and a second transistor; and a read control circuit including a read control transistor, in which the photodiode has a function of supplying charge based on incident light to a gate of the first transistor; the first transistor has a function of storing charge supplied to its gate and converting the charge stored into an output signal; the second transistor has a function of controlling reading of the output signal; the read control transistor functions as a resistor converting the output signal into a voltage signal; and semiconductor layers of the first transistor, the second transistor, and the read control transistor are formed using an oxide semiconductor.

TECHNICAL FIELD

The technical field relates to a photosensor and a driving methodthereof. The technical field also relates to a display device includinga photosensor and a driving method thereof. Further, the technical fieldrelates to a semiconductor device including a photosensor and a drivingmethod thereof.

BACKGROUND ART

In recent years, a display device provided with a light-detecting sensor(also referred to as a photosensor) has attracted attention. In thedisplay device including a photosensor, a display screen also serves asan input region. A display device having an image capture function is anexample of such a display device (see Patent Document 1, for example).

Examples of a semiconductor device including a photosensor are a CCDimage sensor and a CMOS image sensor. Such image sensors are used in,for example, electronic appliances like digital still cameras orcellular phones.

In a display device provided with a photosensor, first, light is emittedfrom the display device. When the light enters a region where an objectexists, the light is blocked by the object, and is partly reflected. Thelight reflected by the object is detected by the photosensor provided ina pixel in the display device. Thus, the existence of the object in thepixel area can be recognized.

In a semiconductor device with a photosensor, light emitted from anobject or external light reflected by the object is detected by thephotosensor directly or is detected after the light is gathered by anoptical lens or the like.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2001-292276

DISCLOSURE OF INVENTION

In a semiconductor device with a photosensor, a circuit with atransistor is formed in each pixel in order that electric signals that aphotosensor in each pixel has generated by detecting light may becollected.

A semiconductor device with a photosensor includes a read controltransistor for converting an output signal based on incident light intoa voltage signal. It is possible to convert an output signal based onincident light into a voltage signal by resistive division of atransistor in a photosensor and a read control transistor being regardedas resistors.

In a semiconductor device with a photosensor, a read control transistorcan be a transistor using amorphous silicon or polycrystalline silicon.However, off-state current, which is current flowing through atransistor being in the off-state, passes through a transistor usingamorphous silicon or polycrystalline silicon. This contributes to thefact that the power consumption of the semiconductor device with aphotosensor increases with time in a period when a read operation is notperformed.

An object of one embodiment of the present invention is to provide asemiconductor device the photosensor of which can achieve low powerconsumption.

One embodiment of the present invention is a semiconductor deviceincluding: a photosensor including a photodiode, a first transistor, anda second transistor; and a read control circuit including a read controltransistor. The photodiode has a function of supplying charge based onincident light to a gate of the first transistor. The first transistorhas a function of storing charge supplied to its gate and converting thecharge stored into an output signal. The second transistor has afunction of controlling reading of the output signal. The read controltransistor functions as a resistor converting the output signal into avoltage signal. Semiconductor layers of the first transistor, the secondtransistor, and the read control transistor are formed using an oxidesemiconductor.

One embodiment of the present invention is a semiconductor deviceincluding: a photosensor including a photodiode, a first transistor, asecond transistor, and a third transistor; and a read control circuitincluding a read control transistor. The photodiode has a function ofsupplying charge based on incident light to a gate of the firsttransistor. The first transistor has a function of storing chargesupplied to its gate and converting the charge stored into an outputsignal. The second transistor has a function of keeping the chargestored on the gate of the first transistor. The third transistor has afunction of controlling reading of the output signal. The read controltransistor functions as a resistor converting the output signal into avoltage signal. Semiconductor layers of the first transistor, the secondtransistor, the third transistor, and the read control transistor areformed using an oxide semiconductor.

One embodiment of the present invention is a semiconductor deviceincluding: a photosensor including a photodiode, a first transistor, anda second transistor; and a read control circuit including a read controltransistor. The photodiode has a function of supplying charge based onincident light to a gate of the first transistor. The first transistorhas a function of storing charge supplied to its gate and converting thecharge stored into an output signal. The second transistor has afunction of controlling reading of the output signal. The read controltransistor functions as a resistor converting the output signal into avoltage signal. Semiconductor layers of the first transistor, the secondtransistor, and the read control transistor are formed using an oxidesemiconductor. A voltage applied to a gate of the read controltransistor is changed in accordance with the output signal, therebychanging resistance of the read control transistor.

One embodiment of the present invention is a semiconductor deviceincluding: a photosensor including a photodiode, a first transistor, asecond transistor, and a third transistor; and a read control circuitincluding a read control transistor. The photodiode has a function ofsupplying charge based on incident light to a gate of the firsttransistor. The first transistor has a function of storing chargesupplied to its gate and converting the charge stored into an outputsignal. The second transistor has a function of keeping the chargestored on the gate of the first transistor. The third transistor has afunction of controlling reading of the output signal. The read controltransistor functions as a resistor converting the output signal into avoltage signal. Semiconductor layers of the first transistor, the secondtransistor, the third transistor, and the read control transistor areformed using an oxide semiconductor. A voltage applied to a gate of theread control transistor is changed in accordance with the output signal,changing resistance of the read control transistor.

In one embodiment of the present invention, a plurality of thephotosensors is formed; the photosensor has a function of performing areset operation, a storage operation, and a read operation; theplurality of the photosensors have a function of concurrently performingthe reset operation, concurrently performing the storage operation, andsequentially performing the read operation.

The semiconductor device refers to an element having a semiconductorproperty and to all the objects having the element. For example, adisplay device having a transistor is simply referred to as asemiconductor device in some cases.

The present invention can provide a semiconductor device thephotosensors of which can achieve low power consumption.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are a circuit diagram and a timing diagram fordescribing one embodiment of the present invention.

FIG. 2 is a circuit diagram for describing one embodiment of the presentinvention.

FIGS. 3A and 3B are a circuit diagram and a timing diagram fordescribing one embodiment of the present invention.

FIG. 4 is a block diagram for describing one embodiment of the presentinvention.

FIG. 5 is a circuit diagram for describing one embodiment of the presentinvention.

FIG. 6 is a timing diagram for describing one embodiment of the presentinvention.

FIG. 7 is a timing diagram for describing one embodiment of the presentinvention.

FIG. 8 is a cross-sectional view for describing one embodiment of thepresent invention.

FIGS. 9A to 9D are diagrams each showing an example of an electronicappliance.

FIGS. 10A and 10B are circuit diagrams for describing one embodiment ofthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be hereinafter described withreference to the drawings. However, the present invention can be carriedout in many different modes, and it is easily understood by thoseskilled in the art that modes and details of the present invention canbe changed in various ways without departing from the purpose and thescope of the present invention. Therefore, the present invention shouldnot be interpreted as being limited to the description of theembodiments. Note that identical portions or portions having the samefunction in all drawings showing the structure of the invention that aredescribed below are denoted by the same reference numerals.

Note that the size, the thickness of a layer, the signal waveform, orthe region of each structure shown in drawings or the like of theembodiments is exaggerated for simplicity in some cases. Therefore, theembodiments of the present invention are not limited to such scales.

Note that terms such as first, second, third to Nth (N is a naturalnumber) employed in this specification are used in order to avoidconfusion between components and do not set a limitation on number.

Embodiment 1

In this embodiment, an example of a semiconductor device which is oneembodiment of the disclosed invention will be described with referenceto FIGS. 1A and 1B and FIG. 2.

FIG. 1A shows an example of the circuit configuration of a photosensor11 included in a semiconductor device 10 and an example of theconfiguration of a read control circuit 12 connected to the photosensor11.

The photosensor 11 includes a photodiode 13, a transistor 14 (alsoreferred to as a first transistor), and a transistor 15 (also referredto as a second transistor). The read control circuit 12 includes atransistor 18 (also referred to as a read control transistor).

In the photosensor 11, one electrode of the photodiode 13 is connectedto a photodiode reset line 16 and the other electrode is connected to agate of the transistor 14. One of a source and a drain of the transistor14 is connected to a photosensor reference high supply line 20, and theother of the source and the drain is connected to one of a source and adrain of the transistor 15. A gate of the transistor 15 is connected toa gate line 17, and the other of the source and the drain of thetransistor 15 is connected to a photosensor output line 22.

In the read control circuit 12, one of a source and a drain of thetransistor 18 is connected to the other of the source and the drain ofthe transistor 15 and the photosensor output line 22, and the other ofthe source and the drain is connected to a photosensor reference lowsupply line 21. A gate of the transistor 18 is connected to aphotosensor selection line 19.

Note that in circuit diagrams in this specification, a transistor havingan oxide semiconductor layer is denoted by a symbol “OS” so that it canbe identified as a transistor having an oxide semiconductor layer. Forexample, in FIG. 1A, the transistor 14, the transistor 15, and thetransistor 18 are transistors having semiconductor layers of an oxidesemiconductor.

The transistor 14, the transistor 15, and the transistor 18 have anoxide semiconductor in their semiconductor layers. When the abovetransistors have semiconductor layers of an oxide semiconductor, theiroff-state current, which is current flowing through a transistor beingin the off state (being off) can be extremely low.

Note that either of the semiconductor layer of the transistor 14 andthat of the transistor 15 is formed using an oxide semiconductor.Specifically, the transistor 14 may be a transistor using an oxidesemiconductor as shown in FIG. 10A; alternatively, the transistor 15 maybe a transistor using an oxide semiconductor as shown in FIG. 10B.Examples of a semiconductor used in the semiconductor layers of thetransistor 14 and the transistor 15 except an oxide semiconductorinclude an amorphous silicon semiconductor, a microcrystalline siliconsemiconductor, a polycrystalline silicon semiconductor, and a singlecrystal silicon semiconductor. The transistor 14 in particular, whichhas a function of converting charge from the photodiode 13 into anoutput signal, preferably uses a single crystal semiconductor to have ahigh mobility.

Note that the meaning of the description that explicitly states “A and Bare connected to each other” includes “A and B are electricallyconnected to each other”, “A and B are functionally connected to eachother”, and “A and B are directly connected to each other”.

A signal is supplied to the photodiode reset line 16 and the gate line17 by a photosensor driver circuit. The photosensor driver circuitsupplies a signal used to perform a reset operation, a storageoperation, and a read operation (also referred to as a selectingoperation), which will be described later, to the photosensor 11 placedin a particular row.

The photosensor selection line 19, the photosensor reference high supplyline 20, the photosensor reference low supply line 21, and thephotosensor output line 22 are connected to a photosensor readingcircuit. The photosensor reading circuit has a function of controllingsignals input to the lines used to read an output signal of thephotosensor 11 in a selected row.

Note that the photosensor reading circuit allows, by using an OPamplifier, an output signal of the photosensor, which is an analogsignal, to be output to an external unit through the photosensor outputline 22 in the form of an analog signal or to be output to an externalunit after being converted into a digital signal by an A/D convertercircuit.

The photodiode 13 can be a PN diode, a PIN diode, a Schottky diode, oran avalanche diode. In the case where a PN diode or a PIN diode is usedas the photodiode 13, applicable diode is that in which semiconductorswith the corresponding conductivity types (a combination of p-typeconductivity and n-type conductivity or a combination of p-typeconductivity, i-type conductivity, and n-type conductivity) are stacked;or that in which semiconductors with the corresponding conductivitytypes lie in the same plane. A semiconductor used in the photodiode 13can be an amorphous semiconductor, a microcrystalline semiconductor, apolycrystalline semiconductor, a single crystal semiconductor, or thelike. The photodiode has a function of generating an electric signal inaccordance with the intensity of light. Incident light on the photodiodeis light reflected by an object or light emitted from an object. Asource of light which is to be reflected by an object can be a lightingdevice included in the semiconductor device or external light.

The transistor 14 has a function of accumulating (storing) charge in itsgate. By the charge stored in the gate, the value of a voltage appliedbetween the gate and a source of the transistor 14 and the value ofresistance between the source and a drain of the transistor 14 arevaried. Then, resistive division between the photosensor reference highsupply line 20 and the photosensor reference low supply line 21 is made,so that the photosensor output line 22 can have a voltage value based onincident light.

The transistor 15 has a function of controlling reading of an outputsignal of the photosensor 11. Specifically, the transistor 15 has afunction of transferring an output signal of the photosensor 11 to thephotosensor output line 22. Therefore, the transistor 15 is required tobe a switch which reads an output signal at high speed by controllingelectrical connection between terminals.

It is preferable that the transistor 14 and the transistor 15 minimize,in a period when a read operation is performed on another photosensorthat is connected to the same photosensor output line 22, a currentflowing between the photosensor output line 22 and the photosensorreference high supply line 20 and thus do not contribute to variationsin the potential of the photosensor output line 22. For this reason, thetransistor 14 and the transistor 15 are preferably transistors with alow off-state current.

In the structure of this embodiment, the transistors used in thephotosensor 11 have semiconductor layers of an oxide semiconductor andthus can have an extremely low off-state current, as shown in FIG. 1A.Thus, these transistors minimize a current flowing between thephotosensor output line 22 and the photosensor reference high supplyline 20 in a period when a read operation is performed on anotherphotosensor that is connected to the same photosensor output line 22 andthus do not contribute to variations in the potential of the photosensoroutput line 22.

Next, the read control circuit 12 will be described. The read controlcircuit 12 shown in FIG. 1A is used for a single column of thephotosensors 11. The read control circuit 12 for a signal row of thephotosensors 11 includes the transistor 18.

In the read control circuit 12, a signal output to the photosensoroutput line 22 can be converted into a voltage signal by resistivedivision between the transistor 14, in which resistance between thesource and the drain varies according to an output signal based onincident light on the photosensor 11, and the transistor 18, in whichresistance between the source and the drain is set by a constant voltagefrom the photosensor selection line 19. In other words, the read controlcircuit 12 serves as a resistor for converting a signal output to thephotosensor output line 22 into a voltage signal.

Note that in this embodiment, in a period except a period when the readcontrol circuit 12 converts a signal output to the photosensor outputline 22 into a voltage signal, it is preferable to achieve low powerconsumption by reducing a current flowing between the photosensor outputline 22 and the photosensor reference low supply line 21 to extremelylow in such a manner that the transistor 18 is turned off by a constantvoltage applied to the transistor 18 through the photosensor selectionline 19.

In the semiconductor device with a photosensor, the transistor 18included in the read control circuit 12 has a semiconductor layer of anoxide semiconductor, and thus has an extremely low off-state current.Further, in a period except a period when the read control circuit 12converts a signal output to the photosensor output line 22 into avoltage signal, it is possible to prevent variations in the potential ofthe photosensor output line 22 by reducing a current flowing between thephotosensor output line 22 and the photosensor reference low supply line21 to extremely low in such a manner that the transistor 18 is turnedoff by a constant voltage applied to the transistor 18 through thephotosensor selection line 19.

Here, a signal to be applied to the photosensor selection line 19 isadjusted so that resistance between the source and the drain of thetransistor 18 may be changed according to the resistance between thesource and the drain of the transistor 14. For example, in order thatthe resistance between the source and the drain of the transistor 14 maybe made high by a large amount of incident light on the photosensor 11,the voltage of the photosensor selection line 19 is made low so that theresistive division between the transistor 14 and the transistor 18 mayyield a voltage signal output to the photosensor output line 22. Incontrast, in order that the resistance between the source and the drainof the transistor 14 may be made low by a small amount of incident lighton the photosensor 11, the voltage of the photosensor selection line 19is made high so that the resistive division between the transistor 14and the transistor 18 may yield a voltage signal output to thephotosensor output line 22. As a result, it is possible to provide aninexpensive semiconductor device capable of capturing, with a highresolution, an image based on a wide range of light intensities.

Further, the semiconductor device with a photosensor according to thisembodiment includes the transistor 14, the transistor 15, and thetransistor 18 which have semiconductor layers of an oxide semiconductor.The off-state current of the transistors having semiconductor layers ofan oxide semiconductor can be extremely low. Therefore, in a period whenthe photosensor does not operate, a flow-through current between thephotosensor reference high supply line 20 and the photosensor referencelow supply line 21 can be extremely low. This leads to lower powerconsumption of the semiconductor device with a photosensor.

Next, the operation of the photosensor 11 shown in FIG. 1A will bedescribed with reference to a timing diagram of FIG. 1B. In FIG. 1B,signals 31 to 35 are the potential of the photodiode reset line 16, thepotential of the gate line 17, the potential of the gate of thetransistor 14, the potential of the photosensor output line 22, and thepotential of the photosensor selection line 19, which are shown in FIG.1A, respectively.

Note that the timing diagram of FIG. 1B includes a reset period when thereset operation is performed, a storage period when the charge storageoperation is performed, and the read period when the read operation isperformed. A period from time A to time B corresponds to the resetperiod. A period from the time B to time C corresponds to the storageperiod. A period from the time C to time D corresponds to the readperiod. A period from the time A to time E corresponds to the readoperation period.

In FIG. 1B, a high potential is “H”, and a low potential is “L”. Notethat in FIG. 1B, a transistor is turned off when a low-level signal isapplied to the gate of the transistor. In addition, in FIG. 1B, when ahigh-level signal is applied to the gate of the transistor, a transistoris put into a conducting state in which resistance between the sourceand the drain of the transistor changes according to the magnitude ofthe applied potential.

At the time A, the potential of the photodiode reset line 16 (the signal31) becomes “H” (the reset operation starts), so that the photodiode 13conducts and the potential of the gate of the transistor 14 (the signal33) becomes “H”. The potential of the gate line 17 (the signal 32)becomes “L”, so that the transistor 15 is turned off. In addition, thepotential of the photosensor selection line 19 (the signal 35) becomes apredetermined value (the read operation starts), allowing the transistor18 to function as a resistor in accordance with a voltage applied to thegate of the transistor 18. Then the transistor 14, the transistor 15,and the transistor 18 function as resistors, so that the transistor 15has a high resistance, causing the photosensor output line 22 (thesignal 34) to be at the same voltage level as the photosensor referencelow supply line 21, i.e., to be at a low voltage level.

At the time B, the photodiode reset line 16 (the signal 31) goes “L”(the reset operation terminates and the storage operation starts), sothat photo-current in the photodiode 13 is increased by incident lightand the potential of the gate of the transistor 14 (the signal 33) isdecreased in accordance with the amount of the incident light. In otherwords, the photodiode 13 has a function of supplying charge to the gateof the transistor 14 in accordance with incident light, and thusdecreases the potential of the gate of the transistor 14. Consequently,resistance between the source and the drain of the transistor 14 ischanged. Then the potential of the gate line 17 (the signal 32) becomes“L”, turning off the transistor 15. Further, the potential of thephotosensor selection line 19 (the signal 35) becomes a predeterminedvalue, allowing the transistor 18 to function as a resistor inaccordance with a voltage applied to the gate of the transistor 18. Thenthe transistor 14, the transistor 15, and the transistor 18 function asresistors, so that the transistor 15 has a high resistance, causing thephotosensor output line 22 (the signal 34) to be at the same voltagelevel as the photosensor reference low supply line 21, i.e., to be at alow voltage value.

At the time C, the potential of the gate line 17 (the signal 32) becomes“H” (the storage operation terminates and the selecting operationstarts), so that the transistor 15 is turned on. Then, the photosensoroutput line 22 has a voltage obtained by resistive division between thetransistor 14 in which resistance between the source and the drainchanges in accordance with incident light; and the transistor 18 inwhich resistance is determined by a constant voltage supplied by thephotosensor selection line 19. Here, the magnitude of the potential ofthe photosensor output line 22 (the signal 34) depends on the speed atwhich the potential of the gate of the transistor 14 (the signal 33)decreases, that is, changes in accordance with the amount of incidentlight on the photodiode 13 during the storage operation. Therefore, theamount of incident light on the photodiode 13 during the storageoperation can be known by obtaining the potential of the photosensoroutput line 22.

At the time D, the potential of the gate line 17 (the signal 32) becomes“L” (the selecting operation terminates), so that the transistor 15 isturned off. In addition, the potential of the photosensor selection line19 (the signal 35) becomes a predetermined value, allowing thetransistor 18 to function as a resistor in accordance with a voltageapplied to the gate of the transistor 18. Then the transistor 14, thetransistor 15, and the transistor 18 function as resistors, so that thetransistor 15 has a high resistance, causing the photosensor output line22 (the signal 34) to be at the same voltage level as the photosensorreference low supply line 21, i.e., to be at a low voltage level.

At the time E, the potential of the photosensor selection line 19 (thesignal 35) is changed from the predetermined value to a value at whichthe transistor 18 is turned off (the read operation terminates). As aresult, turning off the transistor 15 and the transistor 18 reduces aflow-through current caused in a period when the read operation is notperformed and between the photosensor reference high supply line 20 andthe photosensor reference low supply line 21.

Note that the potential of the photosensor selection line 19 (the signal35) needs to be “H” at least in a period when the potential of the gateline 17 (the signal 32) is “H” i.e., a period when the selectingoperation is performed; In FIG. 1B, the potential of the photosensorselection line 19 (the signal 35) may be “L” during the period from thetime A to the time C.

For the above-described configuration shown in FIG. 1A, the position ofthe read control circuit 12 can be changed from the photosensorreference low supply line 21 side to the photosensor reference highsupply line 20 side, as shown in FIG. 2.

Note that when the intensity of incident light on the photodiode 13 ishigh, resistance between the source and the drain becomes high.Consequently, the potential of the photosensor output line 22 (thesignal 34) decreases from “H” on a large scale, and thus becomesapproximately the same value as the potential of the photosensorreference low supply line 21. For this reason, when the intensity of thelight is high, variations in the potential set in accordance with theoutput signal are not distinguished. In this case, the potential of thephotosensor selection line 19 (the signal 35) is reduced, increasingresistance between the source and the drain of the transistor 18; thus,the photosensor output line 22 can have a voltage value which isobtained by resistive division and is based on the incident light.

The above operation has been described assuming that the intensity ofincident light on the photodiode 13 is high, but it can be also used inthe case where the intensity of the incident light on the photodiode islow. When the light is faint, resistance between the source and thedrain is low. Hence, the photosensor output line 22 and the photosensorreference high supply line 20 become approximately at the samepotential, so that variations in a potential determined according to anoutput signal are not distinguished. In this case, the potential of thephotosensor selection line 19 (the signal 35) is increased, increasingresistance between the source and the drain of the transistor 18; thus,the photosensor output line 22 can have a voltage value which isobtained by resistive division and is based on the incident light.

In other words, in this embodiment, resistance between the source andthe drain of the transistor 18 can be easily increased or decreased byincreasing or decreasing the potential of the photosensor selection line19 (the signal 35). This enables a photosensor capable of convertingeither strong light or weak light into an electric signal with accuracy.Consequently, it is possible to provide the semiconductor device 10including a photosensor capable of outputting electric signals on thebasis of a wide range of light intensities.

As described above, the operation of individual photosensors is achievedby repeatedly performing, in the read operation period, the resetoperation, the storage operation, and the selecting operation. As statedabove, it is preferable that the off-state current of the transistor 14,the transistor 15, and the transistor 18 be made extremely low by theuse of an oxide semiconductor. The semiconductor device 10 can achievelow power consumption when its flow-through current which occurs duringnon-operating period is reduced.

Such a semiconductor device including a photosensor can be used in anelectronic appliance such as a scanner or a digital still camera. Inaddition, the semiconductor device including the photosensor can be usedin a display device having a touch panel function.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 2

In this embodiment, an example of the semiconductor device, which is oneembodiment of the disclosed invention, will be described with referenceto FIGS. 3A and 3B. This example has a different structure fromEmbodiment 1. Note that in this embodiment, the same components as thosein FIG. 1A used in Embodiment 1 are denoted by the same referencenumerals as those in FIG. 1A and will not be described again.

The semiconductor device 10 shown in FIG. 3A is different from thatshown in FIG. 1A in having a transistor 41 (also referred to as a thirdtransistor) between the gate of the transistor 14 and the otherelectrode of the photodiode 13. The gate of the transistor 41 isconnected to a gate line 42.

The transistor 41 has a function of controlling the storage operationperformed by the photosensor 11. That is, the transistor 41 has afunction of transferring an electric signal generated by the photodiode13 to the gate of the transistor 14 when being on. In addition, thetransistor 41 has a function of keeping charge accumulated (stored) onthe gate of the transistor 14 when being off. For this reason, atransistor with an extremely low off-state current is desirably used asthe transistor 41.

Therefore, a semiconductor layer forming the channel formation region ofthe transistor 41 preferably is formed using an oxide semiconductor witha very low off-state current and relatively high mobility like thetransistor 14, the transistor 15, and the transistor 18. A transistorformed using an oxide semiconductor has an electrical characteristic ofmuch lower off-state current than a transistor formed using silicon orthe like.

Note that between the semiconductor layers of the transistor 14 and thetransistor 15, either of them is formed using an oxide semiconductor.

Next, the operation of the photosensor 11 shown in FIG. 3A will bedescribed with reference to a timing diagram of FIG. 3B. In FIG. 3B,signals 51 to 56 are the potential of the photodiode reset line 16, thepotential of the gate line 42, the potential of the gate line 17, thepotential of the gate of the transistor 14, the potential of thephotosensor output line 22, and the potential of the photosensorselection line 19, which are shown in FIG. 3A, respectively.

Note that the timing diagram of FIG. 3B includes a reset period when thereset operation is performed, a storage period when the charge storageoperation is performed, and the read period when the read operation isperformed. A period from time A to time B corresponds to the resetperiod. A period from the time B to time C corresponds to the storageperiod. A period from the time D to time E corresponds to the readperiod. A period from the time A to time F corresponds to the readoperation period.

In FIG. 3B, a high potential is “H”, and a low potential is “L”. Notethat in FIG. 3B, a transistor is turned off when a low-level signal isapplied to the gate of the transistor. In addition, in FIG. 3B, when ahigh-level signal is applied to the gate of the transistor, a transistoris put into a conducting state in which resistance between the sourceand the drain of the transistor changes according to the magnitude ofthe applied potential.

At the time A, the potential of the photodiode reset line 16 (the signal51) becomes “H” and the potential of the gate line 42 (the signal 52)becomes “H” (the reset operation starts), so that the photodiode 13 andthe transistor 41 conduct and the potential of the gate of thetransistor 14 (the signal 54) becomes “H”. The potential of the gateline 17 (the signal 53) becomes “L”, thereby turning off the transistor15. In addition, the potential of the photosensor selection line 19 (thesignal 56) becomes a predetermined value (the read operation starts),allowing the transistor 18 to function as a resistor in accordance witha voltage applied to the gate of the transistor 18. Then the transistor14, the transistor 15, and the transistor 18 function as resistors, sothat the transistor 15 has a high resistance, causing the photosensoroutput line 22 (the signal 55) to be at the same voltage level as thephotosensor reference low supply line 21, i.e., to be at a low voltagevalue.

At the time B, the potential of the photodiode reset line 16 (the signal51) becomes “L” and the potential of the gate line 42 (the signal 52)remains to be “H” (the reset operation terminates and the storageoperation starts), so that photo-current in the photodiode 13 isincreased by incident light and the potential of the gate of thetransistor 14 (the signal 54) is decreased in accordance with the amountof the incident light. In other words, the photodiode 13 has a functionof supplying charge to the gate of the transistor 14 in accordance withthe incident light, and thus decreases the potential of the gate of thetransistor 14. Consequently, resistance between the source and the drainof the transistor 14 is changed. Then the potential of the gate line 17(the signal 53) becomes “L”, turning off the transistor 15. Further, thepotential of the photosensor selection line 19 (the signal 56) becomes apredetermined value, allowing the transistor 18 to function as aresistor in accordance with a voltage applied to the gate of thetransistor 18. Then the transistor 14, the transistor 15, and thetransistor 18 function as resistors, so that the transistor 15 has ahigh resistance, causing the photosensor output line 22 (the signal 55)to be at the same voltage level as the photosensor reference low supplyline 21, i.e., to be at a low voltage level.

At the time C, the gate line 42 (the signal 52) goes “L” (the storageoperation terminates), turning off the transistor 41. Then, thepotential of the gate of the transistor 14 (the signal 54) becomesconstant, that is, the amount of charge accumulated (stored) on the gateof the transistor 14 becomes constant. The potential (the amount of thecharge) of the gate of the transistor 14 is determined by the amount ofthe photocurrent generated by the photodiode during the storageoperation. In other words, the potential (the amount of the charge) ofthe transistor 14 is changed in accordance with the intensity of theincident light on the photodiode.

Note that when the gate line 42 (the signal 52) goes “L”, the potential(the amount of the charge) of the gate of the transistor 14 is changedby parasitic capacitance between the gate line 42 and the gate of thetransistor 14. When the amount of change in the potential (the amount ofthe charge) due to the parasitic capacitance is large, the readoperation cannot be performed with accuracy. In order that the amount ofchange in the potential (the amount of the charge) due to the parasiticcapacitance may be reduced, it is effective to take a measure forreducing capacitance between the gate and the source (or between thegate and the drain) of the transistor 41, a measure for increasing thegate capacitance of the transistor 14, a measure for providing the gateline 42 with a storage capacitor, or the like. Note that in FIGS. 3A and3B, these measures are taken and change in the potential (the amount ofthe charge) due to the parasitic capacitance can be thus ignored.

At the time D, the gate line 17 (the signal 53) goes “H” (the selectingoperation starts), so that the transistor 15 is turned on. Then, thephotosensor output line 22 has a voltage obtained by resistive divisionbetween the transistor 14 in which resistance between the source and thedrain changes in accordance with incident light; and the transistor 18in which resistance is determined by a constant voltage supplied by thephotosensor selection line 19. Here, the magnitude of the potential ofthe photosensor output line 22 (the signal 55) depends on the potentialof the gate of the transistor 14 (the signal 54) decreases, that is,changes in accordance with the amount of incident light on thephotodiode 13 during the storage operation. Therefore, the amount ofincident light on the photodiode 13 during the storage operation can beknown by obtaining the potential of the photosensor output line 22.

At the time E, the gate line 17 (the signal 53) goes “L” (the selectingoperation terminates), turning off the transistor 15. In addition, thepotential of the photosensor selection line 19 (the signal 56) becomes apredetermined value, allowing the transistor 18 to function as aresistor in accordance with a voltage applied to the gate of thetransistor 18. Then the transistor 14, the transistor 15, and thetransistor 18 function as resistors, so that the transistor 15 has ahigh resistance, causing the photosensor output line 22 (the signal 55)to have the same voltage level as the photosensor reference low supplyline 21, i.e., to be low.

At the time F, the potential of the photosensor selection line 19 (thesignal 56) is changed from the predetermined value to a value at whichthe transistor 18 is turned off (the read operation terminates). As aresult, turning off the transistor 15 and the transistor 18 reduces aflow-through current between the photosensor reference high supply line20 and the photosensor reference low supply line 21.

Note that the photosensor selection line 19 (the signal 56) needs to go“H” at least in a period when the gate line 17 (the signal 53) is “H”i.e., a period when the selecting operation is performed; In FIG. 3B,the photosensor selection line 19 (the signal 35) may be “L” during theperiod from the time A to the time D.

Note that when the intensity of incident light on the photodiode 13 ishigh, resistance between the source and the drain becomes high.Consequently, the potential of the photosensor output line 22 (thesignal 55) decreases from “H” on a large scale, and thus becomesapproximately the same value as the potential of the photosensorreference low supply line 21. For this reason, when the intensity of thelight is high, variations in the potential set in accordance with theoutput signal are not distinguished. In this case, the potential of thephotosensor selection line 19 (the signal 56) is reduced, increasingresistance between the source and the drain of the transistor 18; thus,the photosensor output line 22 can have a voltage value which isobtained by resistive division and is based on the incident light.

The above operation is described assuming that the intensity of incidentlight on the photodiode 13 is high, but it can be also used in the casewhere the intensity of the incident light on the photodiode is low. Whenthe light is faint, resistance between the source and the drain is low.Hence, the photosensor output line 22 and the photosensor reference highsupply line 20 become approximately at the same potential, so thatvariations in a potential determined according to an output signal arenot distinguished. In this case, the potential of the photosensorselection line 19 (the signal 56) is increased, increasing theresistance of the transistor 18; thus, the photosensor output line 22can have a voltage value which is obtained by resistive division and isbased on the incident light.

In other words, in this embodiment, resistance between the source andthe drain of the transistor 18 can be easily increased or decreased byincreasing or decreasing the potential of the photosensor selection line19 (the signal 56), as in Embodiment 1. This enables a photosensorcapable of converting either strong light or weak light into an electricsignal with accuracy. Consequently, it is possible to provide thesemiconductor device 10 including a photosensor capable of outputtingelectric signals on the basis of a wide range of light intensities.

Unlike Embodiment 1, this embodiment enables the potential of the gateof the transistor 14 in each photosensor to be maintained at a constantvalue even after the storage operation. Specifically, in this embodimentthe semiconductor layer of the transistor 41 is formed using an oxidesemiconductor, allowing the transistor to have an extremely lowoff-state current. In addition, the photosensor 11 can convert incidentlight into an electric signal with accuracy.

As described above, the operation of individual photosensors is achievedby repeatedly performing, in the read operation period, the resetoperation, the storage operation, and the selecting operation. As statedabove, it is preferable that the off-state current of the transistor 14,the transistor 15, and the transistor 18 be made extremely low by theuse of an oxide semiconductor. The semiconductor device 10 can achievelow power consumption when its flow-through current that occurs duringnon-operating period is reduced.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 3

In this embodiment, a display device, which is a semiconductor deviceincluding a photosensor, will be described with reference to FIG. 4 andFIG. 5.

The structure of the display device will be described with reference toFIG. 4. A display device 100 includes a pixel circuit 101, a displayelement control circuit 102, and a photosensor control circuit 103.

The pixel circuit 101 includes a plurality of pixels 104 arranged in amatrix i.e., in the row and column directions. Each of the pixels 104includes a display element 105 and a photosensor 106. The photosensor isnot necessarily provided in each of the pixels 104, and may be providedin every two or more pixels 104. Alternatively, the photosensor may beprovided outside the pixels 104.

The display element control circuit 102 is a circuit controlling thedisplay elements 105, and includes a display element driver circuit 107from which a signal such as video data is input to the display elements105 through signal lines (also referred to as video-data lines or sourcelines); and a display element driver circuit 108 from which a signal isinput to the display elements 105 through scan lines (also referred toas gate lines).

The photosensor control circuit 103 is a circuit controlling thephotosensors 106, and includes a photosensor reading circuit 109 on thesignal lines side and a photosensor driver circuit 110 on the scan linesside.

FIG. 5 shows an example of the circuit configuration of the pixel 104and an example of the configuration of a read control circuit 200connected to the pixel 104. Note that the configuration of photosensorin FIG. 3A is shown in FIG. 5 as an example of the photosensor 106included in the pixel. The read control circuit 200 shows theconfiguration of the photosensor reading circuit in FIG. 3A.

The pixel 104 includes the display element 105 and the photosensor 106.The display element 105 includes a transistor 201, a storage capacitor202, and a liquid crystal element 203.

In the display element 105, a gate of the transistor 201 is connected toa gate line 208; one of a source and a drain of the transistor 201 isconnected to a video-data line 212; the other of the source and thedrain is connected to one electrode of the storage capacitor 202 and oneelectrode of the liquid crystal element 203. The other electrode of thestorage capacitor 202 and the other electrode of the liquid crystalelement 203 are connected to a common wiring supplied with apredetermined potential. The liquid crystal element 203 includes a pairof electrodes and a liquid crystal layer sandwiched between the pair ofelectrodes.

The transistor 201 has a function of controlling injection or ejectionof charge into or from the liquid crystal element 203 and the storagecapacitor 202. For example, when a high potential is applied to the gateline 208, the transistor 201 is turned on and the potential of thevideo-data line 212 is applied to the liquid crystal element 203 and thestorage capacitor 202. The contrast (gray scale) of light passingthrough the liquid crystal element 203 is made by the voltageapplication to the liquid crystal element 203, thereby producing imagedisplay. The storage capacitor 202 has a function of maintaining voltageapplied to the liquid crystal element 203. The display device 100including the liquid crystal element 203 can be a transmissive displaydevice, a reflective display device, or a semi-transmissive displaydevice.

The video-data line 212 is connected to the display element drivercircuit 107 shown in FIG. 4. The display element driver circuit 107 is acircuit which supplies a signal to the display element 105 through thevideo-data line 212. The gate line 208 is connected to the displayelement driver circuit 108 shown in FIG. 4. The display element drivercircuit 108 is a circuit which supplies a signal to the display element105 through the gate line 208. For example, the display element drivercircuit 108 has a function of supplying a signal which selects a displayelement included in a pixel placed in a particular row. The displayelement driver circuit 107 has a function of supplying a signal whichsupplies appropriate potentials to a display element included in a pixelin a selected row.

The semiconductor in the channel formation region of the transistor 201is an oxide semiconductor as described in Embodiments 1 and 2. When thetransistor 201 is allowed to have an extremely low off-state current byan oxide semiconductor, a voltage applied to the liquid crystal element203 can be held more accurately and display quality can be increased. Inaddition, low-power consumption can be achieved by substantiallyreducing the refresh rate by utilization of the transistor 201 being atransistor with an extremely low off-state current.

Although the display element 105 described here includes the liquidcrystal element, the display element 105 may include another elementsuch as a light emitting element. The light emitting element is anelement whose luminance is controlled with current or voltage, andspecific examples thereof include a light emitting diode and an OLED(organic light emitting diode).

The photosensor 106 includes a photodiode 204, a transistor 205 (alsoreferred to as a first transistor), a transistor 206 (also referred toas a third transistor), and a transistor 207 (also referred to as asecond transistor).

In the photosensor 106, one electrode of the photodiode 204 is connectedto the photodiode reset line 210, and the other electrode of thephotodiode 204 is connected to one of the source and the drain of thetransistor 207. One of the source and the drain of the transistor 205 isconnected to the photosensor reference line 213, and the other of thesource and the drain of the transistor 205 is connected to one of thesource and the drain of the transistor 206. The gate of the transistor206 is connected to the gate line 211, and the other of the source andthe drain of the transistor 206 is connected to the photosensor outputline 214. The gate of the transistor 207 is connected to the gate line209. The other of the source and the drain of the transistor 207 isconnected to the gate of the transistor 205 (a node 215).

The gate line 209, the photodiode reset line 210, and the gate line 211are connected to the photosensor driver circuit 110 shown in FIG. 4. Thephotosensor driver circuit 110 has a function of performing the resetoperation, the storage operation, and the read operation, which will bedescribed below, on the photosensor 106 included in a pixel placed in aparticular row.

The photosensor output line 214 and the photosensor reference line 213are connected to the photosensor reading circuit 109 shown in FIG. 4.The photosensor reading circuit 109 has a function of reading an outputsignal from the photosensor 106 included in a pixel in a selected row.

Note that in the photosensor reading circuit 109, an output from thephotosensor which is an analog signal is extracted as an analog signalto an external unit by an OP amplifier; alternatively, an output isconverted into a digital signal by an A/D converter circuit and thenextracted to an external unit.

Note that, the photodiode 204, the transistor 205, the transistor 206,and the transistor 207 of this embodiment which are in the photosensor106, are the same as the photodiode 13, the transistor 14, thetransistor 15, and the transistor 41 which have been described inEmbodiments 1 and 2; therefore, see the above description for theirdetails. Further, the transistor 216 of this embodiment which is in theread control circuit 200 is the same as transistor 18 described inEmbodiments 1 and 2; therefore, see the above description for itsdetails.

This enables a photosensor capable of outputting electric signals on thebasis of a wide range of light intensities. In other words, it ispossible to convert either strong light or weak light into an electricsignal with accuracy.

Consequently, as stated above, in the display device of this embodimentthe off-state current of the transistors is made extremely low by usingan oxide semiconductor for them. The semiconductor device 10 can achievelow power consumption when its flow-through current that occurs duringnon-operating period is reduced. Further, making the off-state currentof the transistors extremely low by using an oxide semiconductor forthem improves a function of maintaining charge stored on the gate of thetransistors. Consequently, the photosensor can convert incident lightinto an electric signal with accuracy.

Although the display device having a display function and including thephotosensor is described in this embodiment, this embodiment can beeasily applied to a semiconductor device including a photosensor, whichdoes not have a display function. That is, the semiconductor device canbe formed by removing the circuits necessary for display, specificallythe display element control circuit 102 and the display element 105,from the display device 100 in this embodiment. Examples of thesemiconductor device including a photosensor include an imaging deviceused in an electronic appliance such as a scanner or a digital stillcamera.

According to this embodiment, it is possible to provide an inexpensivedisplay device or semiconductor device capable of capturing, with a highresolution, an image based on a wide range of light intensities.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 4

In this embodiment, a method for driving a plurality of photosensorswill be described.

First, a driving method shown in the timing diagram of FIG. 6 will bedescribed assuming that it is used for the photosensor in FIG. 3A. FIG.6 is a timing diagram showing the signal waveforms of a signal 601 inputto the photodiode reset line 16 in a photosensor of the first row, asignal 602 input to the photodiode reset line 16 in a photosensor of thesecond row, and a signal 603 input to the photodiode reset line 16 in aphotosensor of the third row, a signal 604 input to the gate line 42 inthe photosensor of the first row, a signal 605 input to the gate line 42in the photosensor of the second row, a signal 606 input to the gateline 42 in the photosensor of the third row, a signal 607 input to thegate line 17 in the photosensor of the first row, a signal 608 input tothe gate line 17 in the photosensor of the second row, a signal 609input to the gate line 17 in the photosensor of the third row, and asignal 621 input to the photosensor selection line 19. A period 610 is aperiod required for one-time image capture. A period 611 is a periodduring which the reset operation is performed in the photosensor of thesecond row; a period 612 is a period during which the storage operationis performed in the photosensor of the second row; a period 613 is aperiod during which the selection operation is performed in thephotosensor of the second row. Thus sequentially driving thephotosensors row by row enables images capturing.

It is found here that the storage operation in the photosensor of eachrow has a time lag. That is, image capture in the photosensor of eachrow is not performed simultaneously, leading to blurring of the imagecaptured. In particular, an image of an object which moves fast iseasily taken to have a distorted shape: if the object moves in adirection from the first row to the third row, an enlarged image istaken leaving a trail behind it; and if the object moves in the oppositedirection, a reduced image is taken.

In order to prevent the time lag of the storage operation in thephotosensor of each row, it is effective that the photosensor of eachrow is sequentially driven in a shorter cycle. In this case, however,the output signal of the photosensor needs to be obtained at very highspeed with an OP amplifier or an AD converter circuit, which causes anincrease in power consumption, and makes it very difficult to obtain animage with high resolution, in particular.

Thus, a driving method shown in the timing diagram of FIG. 5 isproposed. FIG. 7 is a timing diagram showing the signal waveforms of asignal 701 input to the photodiode reset line 16 in the photosensor ofthe first row, a signal 702 input to the photodiode reset line 16 in thephotosensor of the second row, a signal 703 input to the photodiodereset line 16 in the photosensor of the third row, a signal 704 input tothe gate line 42 in the photosensor of the first row, a signal 705 inputto the gate line 42 in the photosensor of the second row, a signal 706input to the gate line 42 in the photosensor of the third row, a signal707 input to the gate line 17 in the photosensor of the first row, asignal 708 input to the gate line 17 in the photosensor of the secondrow, a signal 709 input to the gate line 17 in the photosensor of thethird row, and a signal 721 input to the photosensor selection line 19.A period 710 is a period required for one-time image capture. A period711 is a period during which the reset operation (at the same time inall the rows) is performed in the photosensor of the second row, aperiod 712 is a period during which the storage operation (at the sametime in all the rows) is performed in the photosensor of the second row,and a period 713 is a period during which the selection operation isperformed in the photosensor of the second row.

FIG. 7 is different from FIG. 6 in that the reset operation and thestorage operation are performed in the same period in the photosensorsof all the rows, and after the storage operation, the selectionoperation is sequentially performed in each row without synchronizationwith the storage operation. When the storage operation is performed inthe same period, image capture in the photosensor of each row isperformed simultaneously and an image of an object can be easily takenwith little blur even when the object moves fast. Since the storageoperation is performed at the same time, a driver circuit can beprovided in common for the photodiode reset line 16 of each photosensor.Further, a driver circuit can also be provided in common for the gateline 42 of each photosensor. Such driver circuits provided in common areeffective in reducing the number of peripheral circuits or reducingpower consumption. In addition, the selection operation sequentiallyperformed in each row makes it possible to lower the operation rate ofan OP amplifier or an A/D converter circuit when the output signal ofthe photosensor is obtained. At this time, the total time for theselection operation is preferably longer than the time for the storageoperation, which is particularly effective in the case of obtaining animage with high resolution.

Note that FIG. 7 shows the timing diagram of the method for sequentiallydriving the photosensor of each row; it is also effective tosequentially drive the photosensor only in a certain row in order toobtain an image in a particular region. As a result, a desired image canbe obtained while the operation and power consumption of the OPamplifier or the A/D converter circuit are reduced. Further, a methodfor driving the photosensor of every several rows is also effective.That is, one or some of the plurality of photosensors are driven. As aresult, an image with desired resolution can be obtained while theoperation and power consumption of the OP amplifier or the A/D convertercircuit are reduced.

Note that in order to achieve the aforementioned driving method, thepotential of the gate of the transistor 14 in each photosensor needs tobe kept constant even after the storage operation is completed. Thus,the transistor 41 preferably uses an oxide semiconductor to have anextremely low off-current as described in FIG. 3A.

Thus, it is possible to provide a low-power display device orsemiconductor device which allows a high-resolution image of even afast-moving object to be taken with little blur.

This embodiment can be implemented in appropriate combination with theother embodiments.

Embodiment 5

In this embodiment, a structure and fabrication method of asemiconductor device including a photosensor will be described. FIG. 8is a cross-sectional view of a semiconductor device. Note that thefollowing semiconductor device can be applied to a display device.

FIG. 8 shows an example of a cross-sectional view of the semiconductordevice including a photosensor. In the semiconductor device including aphotosensor shown in FIG. 8, a photodiode 502, a transistor 540, atransistor 503, and a liquid crystal element 505 are formed over asubstrate 501 having an insulating surface (a TFT substrate).

An oxide insulating layer 531, a protective insulating layer 532, aninterlayer insulating layer 533, and an interlayer insulating layer 534are provided over the transistor 503 and the transistor 540. Thephotodiode 502 is formed over the interlayer insulating layer 533. Inthe photodiode 502, a first semiconductor layer 506 a, a secondsemiconductor layer 506 b, and a third semiconductor layer 506 c arestacked in that order over the interlayer insulating layer 533 betweenan electrode layer 541 formed over the interlayer insulating layer 533and an electrode layer 542 formed over the interlayer insulating layer534.

The electrode layer 541 is electrically connected to an electrode layer543 which is formed in the interlayer insulating layer 534, and theelectrode layer 542 is electrically connected to a gate electrode layer545 through the electrode layer 541. The gate electrode layer 545 iselectrically connected to a gate electrode layer of the transistor 540,and the photodiode 502 is electrically connected to the transistor 540.

In order to prevent variations in electric characteristics of thetransistor 503 and the transistor 540 each formed using an oxidesemiconductor layer, which are included in the semiconductor deviceincluding a photosensor, impurities such as hydrogen, moisture, hydroxylgroup, or a hydride (also referred to as a hydrogen compound) whichcause the variations are intentionally removed from the oxidesemiconductor layer. Additionally, the oxide semiconductor layer ishighly purified to become i-type (intrinsic) by supplying oxygen whichis a major component of the oxide semiconductor, which is simultaneouslyreduced in a step of removing impurities.

Therefore, it is preferable that the oxide semiconductor containshydrogen and carriers as little as possible. In the transistor 503 andthe transistor 540, a channel formation region is formed in the oxidesemiconductor layer, in which hydrogen contained therein is removed asmuch as possible to be close to 0 so that the hydrogen concentration isless than or equal to 5×10¹⁹/cm³, preferably, less than or equal to5×10¹⁸/cm³, more preferably, less than or equal to 5×10¹⁷/cm³ or lessthan 5×10¹⁶/cm³, and the carrier concentration is less than 5×10¹⁴/cm³,preferably, less than or equal to 5×10¹²/cm³.

Since a very few (closed to 0) carries exist in the oxide semiconductor,it is preferable that the off-state current of the transistor 503 andthe transistor 540 be as low as possible. The off-state current is acurrent that flows between a source and a drain of a transistor in thecase where a gate voltage between −1 V to −10 V is applied. A currentvalue per 1 μm in a channel width (w) of a transistor formed using anoxide semiconductor, which is disclosed in this specification, is lessthan or equal to 100 aA/μm, preferably, less than or equal to 10 aA/μm,more preferably, less than or equal to 1 aA/μm. Further, since there isno pn junction and no hot carrier degradation, electric characteristicsof the transistor are not adversely affected by them.

Therefore, the above transistors 503 and 540 formed using the oxidesemiconductor layer have an extremely low off-state current and havestable electric characteristics and high reliability.

As the oxide semiconductor film included in each of the transistor 503and the transistor 540, any of a four-component metal oxide such as anIn—Sn—Ga—Zn—O film, a three-component metal oxide such as an In—Ga—Zn—Ofilm, an In—Sn—Zn—O film, an In—Al—Zn—O film, a Sn—Ga—Zn—O film, anAl—Ga—Zn—O film, and a Sn—Al—Zn—O film, or a two-component metal oxidesuch as an In—Zn—O film, a Sn—Zn—O film, an Al—Zn—O film, a Zn—Mg—Ofilm, a Sn—Mg—O film, an In—Mg—O film, an In—O film, a Sn—O film, and aZn—O film can be used. Further, SiO₂ may be contained in the above oxidesemiconductor film.

Note that as the oxide semiconductor film, a thin film expressed byInMO₃(ZnO)_(m) (m>0) can be used. Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Gaand Al, Ga and Mn, Ga and Co, or the like. An oxide semiconductor filmwhose composition formula is represented by InMO₃ (ZnO)_(m) (m>0), whichincludes Ga as M, is referred to as the In—Ga—Zn—O oxide semiconductordescribed above, and a thin film of the In—Ga—Zn—O oxide semiconductoris also referred to as an In—Ga—Zn—O-based non-single-crystal film.

Here, a pin photodiode in which a semiconductor layer having p-typeconductivity as the first semiconductor layer 506 a, a high-resistancesemiconductor layer (i-type semiconductor layer) as the secondsemiconductor layer 506 b, and a semiconductor layer having n-typeconductivity as the third semiconductor layer 506 c are stacked is shownas an example.

The first semiconductor layer 506 a is a p-type semiconductor layer andcan be formed using an amorphous silicon film containing an impurityelement imparting p-type conductivity. The first semiconductor layer 506a is formed with a plasma CVD method with the use of a semiconductorsource gas containing an impurity element belonging to Group 13 (such asboron (B)). As the semiconductor material gas, silane (SiH₄) may beused. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the likemay be used. Further alternatively, an amorphous silicon film which doesnot contain an impurity element may be formed, and then, an impurityelement may be introduced to the amorphous silicon film with the use ofa diffusion method or an ion injecting method. Heating or the like maybe performed after introducing the impurity element with an ioninjecting method or the like in order to diffuse the impurity element.In this case, as a method of forming the amorphous silicon film, anLPCVD method, a chemical vapor deposition method, a sputtering method,or the like may be used. The thickness of the first semiconductor layer506 a is preferably 10 nm to 50 nm.

The second semiconductor layer 506 b is an i-type semiconductor layer(intrinsic semiconductor layer) and is formed with an amorphous siliconfilm. For the formation of the second semiconductor layer 506 b, anamorphous silicon film is formed with a plasma CVD method using asemiconductor material gas. As the semiconductor material gas, silane(SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄,or the like may be used. The second semiconductor layer 506 b may bealternatively formed with an LPCVD method, a chemical vapor depositionmethod, a sputtering method, or the like. The thickness of the secondsemiconductor layer 506 b is preferably 200 nm to 1000 nm.

The third semiconductor layer 506 c is an n-type semiconductor layer andis formed with an amorphous silicon film containing an impurity elementimparting n-type conductivity. The third semiconductor layer 506 c isformed with a plasma CVD method using a semiconductor material gascontaining an impurity element belonging to Group 15 (such as phosphorus(P)). As the semiconductor material gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then, an impurity elementmay be introduced to the amorphous silicon film with the use of adiffusion method or an ion injecting method. Heating or the like may beperformed after the impurity element is introduced with an ion injectingmethod or the like in order to diffuse the impurity element. In thiscase, as a method of forming the amorphous silicon film, an LPCVDmethod, a chemical vapor deposition method, a sputtering method, or thelike may be used. The thickness of the third semiconductor layer 506 cis preferably 20 nm to 200 nm.

The first semiconductor layer 506 a, the second semiconductor layer 506b, and the third semiconductor layer 506 c are not necessarily formedusing an amorphous semiconductor, and they may be formed using apolycrystalline semiconductor or a microcrystalline semiconductor (asemi-amorphous semiconductor (SAS)).

Given the Gibbs free energy, the microcrystalline semiconductor belongsto a metastable state of an intermediate between amorphous and singlecrystalline. That is, the microcrystalline semiconductor film is asemiconductor having a third state which is stable in the free energyand has a short range order and lattice distortion. Columnar-like orneedle-like crystals grow in a normal direction with respect to asubstrate surface. The Raman spectrum of microcrystalline silicon, whichis a typical example of a microcrystalline semiconductor, is shifted toa small wavenumber region below 520 cm⁻¹ which representssingle-crystalline silicon. That is, the peak of the Raman spectrum ofthe microcrystalline silicon exists between 520 cm⁻¹ which representssingle crystal silicon and 480 cm⁻¹ which represents amorphous silicon.In addition, microcrystalline silicon contains hydrogen or halogen of 1atomic % or more in order to terminate a dangling bond. Moreover,microcrystalline silicon contains a rare gas element such as helium,argon, krypton, or neon to further promote lattice distortion, providinga favorable microcrystalline semiconductor film with a higher stability.

The microcrystalline semiconductor film can be formed with ahigh-frequency plasma CVD apparatus with a frequency of several tens ofMHz to several hundreds of MHz or with a microwave plasma CVD methodwith a frequency of greater than or equal to 1 GHz. Typically, themicrocrystalline semiconductor film can be formed using a siliconhydride such as SiH₄, Si₂H₆, SiH₂Cl₂ or SiHCl₃, SiCl₄, or SiF₄, which isdiluted with hydrogen. With a dilution with one or more kinds of raregas elements selected from helium, argon, krypton, or neon in additionto silicon hydride and hydrogen, the microcrystalline semiconductor filmcan be formed. In that case, the flow ratio of hydrogen to the siliconhydride is 5:1 to 200:1, preferably, 50:1 to 150:1, more preferably,100:1. Further, a carbide gas such as CH₄ or C₂H₆, a germanium gas suchas GeH₄ or GeF₄, F₂, or the like may be mixed into the gas containingsilicon.

In addition, since the mobility of holes generated by the photoelectriceffect is lower than that of electrons, a pin photodiode has bettercharacteristics when a surface on the p-type semiconductor layer side isused as a light-receiving plane. Here, an example where the photodiode502 receives light from a surface of the substrate 501, over which a pinphotodiode is formed, and the light is converted into electric signalswill be described. Further, light from the semiconductor layer having aconductivity type opposite to that of the semiconductor layer on thelight-receiving plane is disturbance light; therefore, the electrodelayer is preferably formed using a light-blocking conductive film. Asurface on the n-type semiconductor layer side can alternatively be usedas the light-receiving plane.

The liquid crystal element 505 includes a pixel electrode 507, liquidcrystal 508, and a counter electrode 509. The pixel electrode 507 isformed over the substrate 501 and electrically connected to thetransistor 503 through a conductive film 510. The counter electrode 509is formed over a substrate 513 (a counter substrate). The liquid crystal508 is interposed between the pixel electrode 507 and the counterelectrode 509. The transistor 503 corresponds to the transistor 201 inEmbodiment 1.

A cell gap between the pixel electrode 507 and the counter electrode 509can be controlled by using a spacer 516. In FIG. 8, the cell gap iscontrolled by using the columnar spacer 516 selectively formed byphotolithography. Alternatively, the cell gap can be controlled bydispersing spherical spacers between the pixel electrode 507 and thecounter electrode 509.

The liquid crystal 508 is surrounded by a sealing material between thesubstrate 501 and the substrate 513. The liquid crystal 508 may beinjected with a dispenser method (droplet method) or a dipping method(pumping method).

For the pixel electrode 507, a light-transmitting conductive materialsuch as indium tin oxide (ITO), indium tin oxide containing siliconoxide (ITSO), organic indium, organic tin, zinc oxide (ZnO), indium zincoxide (IZO) containing zinc oxide (ZnO), zinc oxide containing gallium(Ga), tin oxide (SnO₂), indium oxide containing tungsten oxide, indiumzinc oxide containing tungsten oxide, indium oxide containing titaniumoxide, indium tin oxide containing titanium oxide, or the like can beused. A conductive composition containing a conductive macromolecule(also referred to as a conductive polymer) can be used to form the pixelelectrode 507. As the conductive macromolecule, a so-called π-electronconjugated conductive polymer can be used. For example, polyaniline or aderivative thereof, polypyrrole or a derivative thereof, polythiopheneor a derivative thereof, a copolymer of two or more of aniline, pyrrole,and thiophene or a derivative thereof, or the like can be given.

Since the transparent liquid crystal element 505 is given as an examplein this embodiment, the light-transmitting conductive material describedabove can be used also for the counter electrode 509 as in the case ofthe pixel electrode 507.

An alignment film 511 is provided between the pixel electrode 507 andthe liquid crystal 508, and an alignment film 512 is provided betweenthe counter electrode 509 and the liquid crystal 508. The alignment film511 and the alignment film 512 can be formed using an organic resin suchas a polyimide or polyvinyl alcohol. Alignment treatment such as rubbingis performed on their surfaces in order to align liquid crystalmolecules in certain direction. Rubbing can be performed by rolling aroller wrapped with cloth of nylon or the like while applying pressureon the alignment film so that the surface of the alignment film isrubbed in certain direction. Note that by using an inorganic materialsuch as silicon oxide, the alignment film 511 and the alignment film 512each having an alignment property can be directly formed with anevaporation method without performing an alignment treatment.

Further, a color filter 514 through which light in a particularwavelength range can pass is formed over the substrate 513 so as tooverlap with the liquid crystal element 505. The color filter 514 can beselectively formed by photolithography after application of an organicresin such as an acrylic-based resin in which colorant is dispersed onthe substrate 513. Alternatively, the color filter 514 can beselectively formed by etching after application of a polyimide-basedresin in which colorant is dispersed on the substrate 513.Alternatively, the color filter 514 can be selectively formed with adroplet discharge method such as an ink-jet method.

Furthermore, a shielding film 515 which can block light is formed overthe substrate 513 so as to overlap with the photodiode 502. By providingthe shielding film 515, light from a backlight that passes through thesubstrate 513 and enters the touch panel can be prevented from beingdirectly delivered to the photodiode 502. Further, disclination due todisorder of alignment of the liquid crystal 508 among pixels can beprevented from being viewed. An organic resin containing black colorantsuch as carbon black or titanium lower oxide whose oxidation number issmaller than that of titanium dioxide can be used for the shielding film515. Alternatively, a film formed using chromium can be used for theshielding film 515.

Furthermore, a polarizing plate 517 is provided on a surface which is onthe opposite side of a surface of the substrate 501 over which the pixelelectrode 507 is formed, and a polarizing plate 518 is provided on asurface which is on the opposite side of a surface of the substrate 513on which the counter electrode 509 is formed.

With the use of an insulating material, the oxide insulating layer 531,the protective insulating layer 532, the interlayer insulating layer533, and the interlayer insulating layer 534 can be formed, depending onthe material, by a method such as a sputtering method, an SOG method, aspin coating, a dip coating, a spray coating, a droplet discharge method(e.g., an ink-jet method, screen printing, offset printing, or thelike), or with a tool such as a doctor knife, a roll coater, a curtaincoater, a knife coater, or the like.

As the oxide insulating layer 531, a single layer or a stack of an oxideinsulating layer such as a silicon oxide layer, a silicon oxynitridelayer, an aluminum oxide layer, an aluminum oxynitride layer, or thelike can be used.

As an inorganic insulating material of the protective insulating layer532, a single layer or a stacked layer of a nitride insulating layersuch as a silicon nitride layer, a silicon nitride oxide layer, analuminum nitride layer, an aluminum nitride oxide layer, or the like canbe used. High-density plasma CVD with the use of microwaves (2.45 GHz)is preferably employed because formation of a dense and high-qualityinsulating layer having high withstand voltage is possible.

To reduce surface roughness, an insulating layer functioning as aplanarization insulating film is preferably used as the interlayerinsulating layers 533 and 534. The interlayer insulating layers 533 and534 can be formed using an organic insulating material having heatresistance such as a polyimide, an acrylic resin, abenzocyclobutene-based resin, a polyamide, or an epoxy resin. Other thansuch organic insulating materials, it is possible to use a single layeror stacked layers of a low-dielectric constant material (a low-kmaterial), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like.

Light from the backlight passes through the substrate 513 and the liquidcrystal element 505 and is delivered to an object 521 on the substrate501 side as shown by an arrow 520. Then, light reflected by the object521 enters the photodiode 502 as shown by an arrow 522.

The liquid crystal element may be a TN (twisted nematic) mode liquidcrystal element, a VA (vertical alignment) mode liquid crystal element,an OCB (optically compensated birefringence) mode liquid crystalelement, an IPS (in-plane switching) mode liquid crystal element, or thelike. Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used.

Note that although the liquid crystal element 505 in which the liquidcrystal 508 is interposed between the pixel electrode 507 and thecounter electrode 509 is described as an example in this embodiment, thesemiconductor device according to one embodiment of the presentinvention is not limited to this structure. A liquid crystal element inwhich a pair of electrodes is formed on the substrate 501 side like anIPS mode liquid crystal element may also be employed.

The above-described structure enables the semiconductor device capableof image capture with high-speed operation.

This embodiment can be implemented in appropriate combination with theother embodiments.

Embodiment 6

The semiconductor device according to one embodiment of the presentinvention can be used as a touch panel. Touch panels can be used indisplay devices, laptop computers, and image reproducing devicesprovided with recording media (a typical example of which is devicesreproducing the content of recording media such as DVDs (digitalversatile disc) and having a display for displaying the reproducedimages). Examples of an electronic appliance in which a touch panelaccording to one embodiment of the present invention can be used includemobile phones, portable game consoles, personal digital assistants,e-book readers, video cameras, digital still cameras, goggle-typedisplays (head mounted displays), navigation systems, audio reproducingdevices (e.g., car audio components and digital audio players), copiers,facsimiles, printers, multifunction printers, automated teller machines(ATM), and vending machines.

In this embodiment, examples of the electronic appliance including atouch panel according to one embodiment of the present invention will bedescribed with reference to FIGS. 9A to 9D.

FIG. 9A shows a display device including a housing 5001, a display area5002, a supporting base 5003, and the like. A touch panel according toone embodiment of the present invention can be used for the display area5002. The use of the touch panel according to one embodiment of thepresent invention for the display area 5002 can provide a display devicecapable of obtaining an image data with high resolution and beingequipped with higher-performance applications. Note that a displaydevice includes all display devices for displaying information, such asdisplay devices for personal computers, for receiving televisionbroadcast, and for displaying advertisement, in its category.

FIG. 9B shows a personal digital assistant including a housing 5101, adisplay area 5102, a switch 5103, a control key 5104, an infrared port5105, and the like. A touch panel according to one embodiment of thepresent invention can be used for the display area 5102. The use of thetouch panel according to one embodiment of the present invention for thedisplay area 5102 can provide a personal digital assistant capable ofobtaining an image data with high resolution and being equipped withhigher-performance applications.

FIG. 9C shows an automated teller machine including a housing 5201, adisplay area 5202, a coin slot 5203, a bill slot 5204, a card slot 5205,a bankbook slot 5206, and the like. A touch panel according to oneembodiment of the present invention can be used for the display area5202. The use of the touch panel according to one embodiment of thepresent invention for the display area 5202 can provide an automatedteller machine capable of obtaining an image data with high resolutionand being equipped with higher-performance applications. The automatedteller machine using the touch panel according to one embodiment of thepresent invention can read information of living body such as afingerprint, a face, a handprint, a palm print, a pattern of hand veins,an iris, and the like which are used for biometrics with higheraccuracy. Therefore, a false non-match rate which is false recognitionof a person to be identified as a different person and a falseacceptance rate which is false recognition of a different person as aperson to be identified can be suppressed.

FIG. 9D shows a portable game console including a housing 5301, ahousing 5302, a display area 5303, a display area 5304, a microphone5305, a speaker 5306, a control key 5307, a stylus 5308, and the like. Atouch panel according to one embodiment of the present invention can beused for the display area 5303 or the display area 5304. The use of thetouch panel according to one embodiment of the present invention for thedisplay area 5303 or the display area 5304 can provide a portable gameconsole capable of obtaining an image data with high resolution andbeing equipped with higher-performance applications. Note that althoughthe portable game console shown in FIG. 9D includes two display areas5303 and 5304, the number of display areas included in the portable gameconsole is not limited thereto.

This embodiment can be implemented in appropriate combination with theother embodiments.

This application is based on Japanese Patent Application serial no.2010-056526 filed with Japan Patent Office on Mar. 12, 2010, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a photosensor comprising: aphotodiode; a first transistor having a gate, the gate of the firsttransistor being connected to one electrode of the photodiode; a secondtransistor having a source and a drain, one of the source and the drainof the second transistor being connected to one of a source and a drainof the first transistor; and a read control circuit including a readcontrol transistor having a source and a drain, one of the source andthe drain of the read control transistor being connected to the other ofthe source and the drain of the second transistor, wherein at least oneof the first transistor, the second transistor, and the read controltransistor includes an oxide semiconductor in a semiconductor layer. 2.The semiconductor device according to claim 1, wherein a voltage appliedto a gate of the read control transistor changes in accordance withincident light so that resistance of the read control transistorchanges.
 3. The semiconductor device according to claim 1, wherein theoxide semiconductor comprises at least one of indium, gallium, tin, andzinc.
 4. The semiconductor device according to claim 1, wherein theother of the first transistor, the second transistor, and the readcontrol transistor includes any one of an amorphous semiconductor, amicrocrystalline semiconductor, a polycrystalline semiconductor, and asingle crystal semiconductor in a semiconductor layer.
 5. Thesemiconductor device according to claim 1, wherein the photodiode isselected from a PN diode, a PIN diode, and a Schottky diode.
 6. Thesemiconductor device according to claim 1, wherein a plurality of thephotosensors according to claims 1 is arranged in row and columndirections over a substrate.
 7. The semiconductor device according toclaim 6, wherein the photosensor is configured to perform a resetoperation, a storage operation, and a read operation.
 8. Thesemiconductor device according to claim 7, wherein the plurality of thephotosensors is configured to concurrently perform the reset operationand the storage operation in all the rows, and sequentially perform theread operation in each row.
 9. A semiconductor device comprising: aphotosensor comprising: a photodiode; a first transistor having a sourceand a drain, one of the source and the drain of the first transistorbeing connected to one electrode of the photodiode; a second transistorhaving a gate, the gate of the second transistor being connected to theother of the source and the drain of the first transistor; a thirdtransistor having a source and a drain, one of the source and the drainof the third transistor being connected to one of a source and a drainof the second transistor; and a read control circuit including a readcontrol transistor having a source and a drain, one of the source andthe drain of the read control transistor being connected to the other ofthe source and the drain of the third transistor, wherein at least oneof the first transistor, the second transistor, the third transistor,and the read control transistor includes an oxide semiconductor in asemiconductor layer.
 10. The semiconductor device according to claim 9,wherein a voltage applied to a gate of the read control transistorchanges in accordance with incident light so that resistance of the readcontrol transistor changes.
 11. The semiconductor device according toclaim 9, wherein the oxide semiconductor comprises at least one ofindium, gallium, tin, and zinc.
 12. The semiconductor device accordingto claim 9, wherein the other of the first transistor, the secondtransistor, the third transistor, the read control transistor includesany one of an amorphous semiconductor, a microcrystalline semiconductor,a polycrystalline semiconductor, and a single crystal semiconductor in asemiconductor layer.
 13. The semiconductor device according to claim 9,wherein the photodiode is selected from a PN diode, a PIN diode, and aSchottky diode.
 14. The semiconductor device according to claim 9,wherein a plurality of the photosensors according to claim 9 is arrangedin row and column directions over a substrate.
 15. The semiconductordevice according to claim 14, wherein the photosensor is configured toperform a reset operation, a storage operation, and a read operation.16. The semiconductor device according to claim 15, wherein theplurality of the photosensors is configured to concurrently perform thereset operation and the storage operation in all the rows, andsequentially perform the read operation in each row.
 17. A semiconductordevice comprising: a photosensor comprising: a photodiode; a firsttransistor having a gate, the gate of the first transistor beingconnected to one electrode of the photodiode; a second transistor havinga source and a drain, one of the source and the drain of the secondtransistor being connected to one of a source and a drain of the firsttransistor; and a read control circuit including a read controltransistor having a source and a drain, one of the source and the drainof the read control transistor being connected to the other of thesource and the drain of the first transistor, wherein at least one ofthe first transistor, the second transistor, and the read controltransistor includes an oxide semiconductor in a semiconductor layer. 18.The semiconductor device according to claim 17, wherein a voltageapplied to a gate of the read control transistor changes in accordancewith incident light so that resistance of the read control transistorchanges.
 19. The semiconductor device according to claim 17, wherein theoxide semiconductor comprises at least one of indium, gallium, tin, andzinc.
 20. The semiconductor device according to claim 17, wherein theother of the first transistor, the second transistor, and the readcontrol transistor includes any one of an amorphous semiconductor, amicrocrystalline semiconductor, a polycrystalline semiconductor, and asingle crystal semiconductor in a semiconductor layer.
 21. Thesemiconductor device according to claim 17, wherein the photodiode isselected from a PN diode, a PIN diode, and a Schottky diode.
 22. Thesemiconductor device according to claim 17, wherein a plurality of thephotosensors according to claim 17 is arranged in row and columndirections over a substrate.
 23. The semiconductor device according toclaim 22, wherein the photosensor is configured to perform a resetoperation, a storage operation, and a read operation.
 24. Thesemiconductor device according to claim 23, wherein the plurality of thephotosensors is configured to concurrently perform the reset operationand the storage operation in all the rows, and sequentially perform theread operation in each row.
 25. A semiconductor device comprising: aphotosensor comprising: a photodiode; a first transistor having a sourceand a drain, one of the source and the drain of the first transistorbeing connected to one electrode of the photodiode; a second transistorhaving a gate, the gate of the second transistor being connected to theother of the source and the drain of the first transistor; a thirdtransistor having a source and a drain, one of the source and the drainof the third transistor being connected to one of a source and a drainof the second transistor; and a read control circuit including a readcontrol transistor having a source and a drain, one of the source andthe drain of the read control transistor being connected to the other ofthe source and the drain of the second transistor, wherein at least oneof the first transistor, the second transistor, the third transistor,and the read control transistor includes an oxide semiconductor in asemiconductor layer.
 26. The semiconductor device according to claim 25,wherein a voltage applied to a gate of the read control transistorchanges in accordance with incident light so that resistance of the readcontrol transistor changes.
 27. The semiconductor device according toclaim 25, wherein the oxide semiconductor comprises at least one ofindium, gallium, tin, and zinc.
 28. The semiconductor device accordingto claim 25, wherein the other of the first transistor, the secondtransistor, the third transistor, the read control transistor includesany one of an amorphous semiconductor, a microcrystalline semiconductor,a polycrystalline semiconductor, and a single crystal semiconductor in asemiconductor layer.
 29. The semiconductor device according to claim 25,wherein the photodiode is selected from a PN diode, a PIN diode, and aSchottky diode.
 30. The semiconductor device according to claim 25,wherein a plurality of the photosensors according to claim 25 isarranged in row and column directions over a substrate.
 31. Thesemiconductor device according to claim 30, wherein the photosensor isconfigured to perform a reset operation, a storage operation, and a readoperation.
 32. The semiconductor device according to claim 31, whereinthe plurality of the photosensors is configured to concurrently performthe reset operation and the storage operation in all the rows, andsequentially perform the read operation in each row.